High-voltage machines in the form of generators are typically used for generating electrical energy. EP 1 981 150 A2 describes a generator comprising a rotatable rotor and a stator arranged around the rotor. The rotor has a rotationally symmetrical laminate stack, in which electrically conductive winding bars extend. An end winding adjoins the laminate stack at both ends and connects the winding bars via connecting webs to form a closed winding.
During operation of high-voltage machines at powers of over 500 MVA, rated voltages of over 10 kV can be reached. The components are subjected to correspondingly high mechanical, thermal and electrical loads. In particular, the winding bars extending in the laminate stack are therefore provided with an electrical insulation system, which is intended to prevent wear owing to electrical partial discharges.
EP 2 521 247 A1 describes an electrical insulation system for winding bars in high-voltage machines, in which each winding bar is enveloped by ground insulation. In addition, poorly conductive internal potential grading is provided between the ground insulation and the winding bars. The ground insulation is furthermore enveloped by corona shielding, which comprises outer corona shielding and end corona shielding.
Owing to the corona shielding and the internal potential grading, the ground insulation is shielded from cavities and detachments. Thus, the electrical potential in the ground insulation is decreased starting from the internal potential grading in the radial direction down to the outer corona shielding. In the region of the point at which the winding bars emerge from the laminate stack, the outer corona shielding ends, while the ground insulation is continued in the direction of the end winding. The electrical field also has a tangential component parallel to the interface with the ground insulation, in addition to the radial component, in this region. The greatest field strength in this case occurs at the end of the outer corona shielding.
It is therefore necessary to ensure potential grading at the end of the outer corona shielding and an increase in the dielectric strength in the vicinity of the exposed ground insulation. This is achieved by the end corona shielding. The aim of the potential grading in this case is to make the tangential potential buildup along the surface of the ground insulation more uniform. In the region of the end corona shielding, the potential is successively built up until it is at a high voltage in the region of the end winding. This potential profile is realized at both bar ends and is thus built up symmetrically.
In order to form the corona shielding, further layers in the form of paints are applied to the ground insulation. The paints for the end corona shielding and the outer corona shielding each contain different fillers, such as graphite and silicon carbide.